Analysis of the cbbXYZ operon in Rhodobacter sphaeroides

Abstract

Three genes, cbbX, cbbY, and cbbZ were found downstream from the form I ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) genes of Rhodobacter sphaeroides. As in chemoautotrophic bacteria, cbbZ was shown to encode phosphoglycolate phosphatase (PGP), whereas the identities of cbbX and cbbY are not known. To determine the physiological function of the cbbXYZ gene products, we constructed R. sphaeroides strains in which the genes were inactivated and characterized the resultant mutant strains according to growth phenotype and levels of RubisCO and PGP. Only a mutation in cbbX resulted in a discernible phenotype, namely, impaired photoautotrophic growth. No PGP activity was observed in any of the mutants, suggesting that the three genes are transcriptionally linked. Studies with a spontaneous chemoautotrophic competent derivative of the CBBX mutant suggested that the cbbXYZ gene products are not essential for chemoautotrophic growth. PGP activity determined in the wild-type strain grown under a variety of growth conditions, and in various strains containing mutations in Calvin-Benson-Bassham cycle structural and regulatory genes, indicated that transcription of the cbb I operon influenced expression of the downstream cbbXYZ operon.Purple nonsulfur photosynthetic bacteria have the capacity to grow under a diverse range of culture conditions, in which CO 2 fixation via the Calvin-Benson-Bassham (CBB) reductive pentose phosphate cycle has been shown to play varied roles. During photo-and chemoautotrophic growth, all of the cellular carbon is derived from CO 2 . Accordingly, during autotrophic growth, key Calvin cycle enzymes, such as ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and phosphoribulokinase, are maximally induced (6, 13, 32). On the other hand, during photoheterotrophic growth, CO 2 fixation functions in a different capacity, fulfilling the role of electron acceptor in the presence of excess reducing potential derived from fixed organic substrates, such as malic or butyric acid, and intermediate levels of RubisCO and phosphoribulokinase are observed (16,32). Alternate electron acceptors, such as dimethyl sulfoxide, can replace the need for CO 2 fixation under these growth conditions, leading to repressed synthesis of CBB cycle enzymes (14,15,34).In Rhodobacter sphaeroides, several enzymes of the CBB cycle are duplicated, and their genes are organized within two physically distinct operons, cbb I and cbb II (2,7,8), both of which are part of the cbb regulon (6, 13). Expression of genes within both cbb operons is positively regulated and coordinated through a common regulatory protein, CbbR, encoded by the cbbR gene, which is divergently transcribed from the cbb I operon (12). In various cbb mutants, CbbR is able to mediate a compensatory expression of genes within one operon in response to disruption of the second operon, thereby maintaining induced levels of key CBB cycle enzymes. Environmental factors affecting synthesis of cbb genes include CO 2 , O 2 , and reduced carbon sources, amo...

Insights into the Mechanism of Catalysis by the P−C Bond-Cleaving Enzyme Phosphonoacetaldehyde Hydrolase Derived from Gene Sequence Analysis and Mutagenesis

“…In addition, possible structural relationships existing between "subgroups" of these proteins have also been identified by other workers (42)(43)(44)(45). The findings from these independent studies are further evidence that the proteins we have grouped together in the phosphonatase family are indeed related.…”

Ribulose bisphosphate carboxylase activity and a Calvin cycle gene cluster in Sulfobacillus species

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Evolutionary Genomics of the HAD Superfamily: Understanding the Structural Adaptations and Catalytic Diversity in a Superfamily of Phosphoesterases and Allied Enzymes

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